The rotary drilling process creates a borehole in the earth by use of a drill bit which is attached to a drill stem, The drill bit and drill stem are lowered and rotated into the earth creating a bore hole by breaking, abrading and fracturing the earth beneath the drill bit. During this process drilling fluid is circulated by means of a pump down the inside of the drill stem and up the annular space between the outside of the drill stem and the wall of the bore hole, the drilling mud is typically a mixture of water and clay, but other drilling muds such as diesel oil, foam, and air have been used. Materials which are products of the drilling process such as rock chips (cuttings and cavings), pieces of casing, cement, the drill stem, hydrocarbon gases such as, but not limited to methane, ethane, propane, other gases associated with hydrocarbon gases such as, but not limited to carbon dioxide and hydrogen sulfide and oil are entrained in the drilling fluid as it circulates from the drill bit up the annular space between the outside of the drill stem and the borehole wall.
Other functions of the drilling fluid include cooling and lubricating the bit and maintaining a hydrostatic pressure on the bore hole which is greater than the pressure in the earth. This hydrostatic pressure prevents uncontrolled flows of oil, gas and water from the earth into the borehole.
Gas mudlogging is a technique for determining the amount and constituent components of the gas which entrained in the drilling fluid. Heretofore, gas was sampled by a mechanically agitated device called a gas trap, which was placed in the shale shaker box. The gas extracted from the gas trap was mixed with air and drawn though tubing to a gas analyzer. The typical gas analyzers used in mudlogging are flame ionization detectors (FID), catalytic combustion detectors (hot wires) and thermal conductivity detectors (TDC). Constituent components of the entrained gas are typically analyzed by chromatography.
Difficulties in assuring good gas sample quality and consistency have been known and studied for many years. The major difficulty of gas sampling has involved the placement and efficiency of the gas trap. Placement within the shaker box and depth of placement of the gas trap in the drilling mud caused major variations in sampling efficiency.
During drilling the shale shaker is often bypassed and the mud level in the shaker box drops to a level which causes the gas trap to be out of the drilling mud. Common reasons for bypassing the shale are to build viscosity by allowing drilled native clay to be recirculated, allowing lost circulation material which is larger than the shale shaker screen to remain in the drilling mud and the failure of gates in the mud tanks to close properly.
Conversely, slugs of drilling mud caused by raising and lowering the drill stem, air trapped in the drill stem and gas, water or oil entering the well bore can cause the level in the shaker box to rise rapidly causing a blockage of the gas trap exhaust port, covering the motor which drives the mechanical agitator, and the drawing of drilling mud into the sample tubing. Drilling mud in the sample tubing causes blockage, damage to the gas analyzers which are designed for a gas environment, and variation in the gas sample rate which adversely affects the accurate measurement of the gas.
Gas traps are typically designed with a mechanical agitator which is rotated by an electric or air driven motor. The environment above the shale shaker box is rated as Class 1, Division 1, Group D by the American Pertroleum Institute. In these environments ignitible concentrations of flammable gases or vapors can exist under normal operating conditions. This requires special procedures and equipment. The agitator with electrical motors must be rated as explosion proof. Many gas traps use air motors to rotate the agitators. This practice reduces the potential danger of electrical sparking. However bearing failure is common in both electrical and air motors and this can cause sparking.
Electrical gas trap motors are turned off when the electrical generator is turned off for repair and maintenance. Smaller drilling rigs typically turn off the electricity during day light hours to save on expenses.
Air motor gas traps are turned off when the rig compressor is turned off for repair and maintenance. Air motors which are driven by rig compressors do not rotate at a constant speed due to fluctuations in air use on the rig. During cold weather water in the air supply often freezes causing the air motor and agitator to stop rotating.
During cold weather drilling mud and condensed water in the gas sample tubing freeze and stop or constrict the gas sample flow. The shale shaker box and the gas trap are usually not enclosed, insulated or heated in cold weather.
The placement of the gas trap in the shale shaker box and the ancillary tubing lines and hoses are often moved by drilling rig personnel during normal maintenance of the shale shaker, desilters, desander, degasser and other drilling mud conditioning equipment. Often the drilling rig personnel disconnect or alter the gas trap configuration to accomplish normal rig maintenance. This adversely affects the quality of the gas sample.